Modern electronic-based imaging systems have a wide variety of uses, including radio telescopes and digital cameras. At a very high level, these devices process wavelengths of electromagnetic radiation to form a composite image. A common device used in such imaging devices is a focal plane array (FPA) of individual antennas, or pixels. The FPA combines radiation from throughout electromagnetic spectrum to form an image.
An FPA is often implemented as a phased array where signals from individual antenna elements are synthesized to form a desired beam, or image. Advanced phased array applications such as the formation of multiple beams, increased resolution at greater distance, and reduced sampling time for imaging systems are achieved by increasing the number of antennas, or the frontend element count. In order for large arrays to be realized, it is preferable to compensate for the increase in element count by a reduction in size, weight and cost so as to keep the overall device within advantageous dimensions. Operating phased arrays at higher frequencies (smaller wavelengths) allows for the reduction in size and weight naturally through utilizing the scaling of components with ever decreasing wavelength. While this has been demonstrated with the relative size of high frequency monolithic microwave integrated circuits (MMICs), this benefit has gone largely unrealized for phased array systems; while the size of the circuits has decreased, fixed aspects of split-block waveguide housing have not led to a reduction of overall system size. Mechanical pins, flange connections, and DC bias have caused the size of a split-block package to remain roughly constant regards of frequency, at roughly 1 in3 from 200-700 GHz.
Recent demonstrations of solid state integrated circuits operating in the deep millimeter wave, sub-millimeter wave, and THz frequencies have provided an impetus towards envisioning imaging systems operating at these frequencies.
Sub-millimeter wavelength (SMMW) imaging systems have the potential to be used for surveillance imaging through airborne obscurants, such as smoke, fog, and dust. SMMW devices operate at frequencies between 0.3 to 3 terahertz (THz). Due to its extremely short wavelength, terahertz radiation can pass through clothing, paper, cardboard, wood, masonry, plastic and ceramics. This makes it ideal for imaging systems that provide discrete detection of remote concealed weapons and improved explosive devices (IEDs).
Current SMMW imaging systems are not practical for battlefield or airborne accommodation due to size and power consumption. One example uses a W-band (75-110 gigahertz) FPA with 1040 elements may be 250 cm2 in size with a total die area of 14,000 mm2, direct current (DC) power requirements of 250 W, a package weight of 15 lbs, and require water cooling. While a SMMW imaging system is desirable, the size is too large for many applications, such as a man-portable system.
Antenna element spacing on the order of a wavelength at the frequency of the system operation is desirable for high frequency compact antenna arrays. Standard waveguide blocks are insufficient to meet these targets. A technology capable of fulfilling the miniaturization promises of high frequencies such as those used for SMMW applications is that of wafer level packaging (WLP), which can bring together different active technologies with etched waveguides in wafer(s) (e.g., silicon, glass, or quartz wafer) for small, dense packages by bonding entire wafers of different wafer substrates. In traditional wafer level packaging, entire active and waveguide wafers are bonded together to create 3-dimensional integrated structures.
Previous advances in wafer level packaging (WLP) allowed for simultaneous integration of waveguide wafer(s) and microwave monolithic integrated circuits (MMICs) wafer(s) to be combined for small size and weight, low loss, and high circuit performance. Despite success from microwave frequencies, to W-band, and even near THz frequencies, some fundamental challenges remain to expanding this type of integration to a robust, low cost process. First, in order to accommodate the waveguide area, the MMIC wafer area is not fully used for MMIC circuits, thus wasting a portion of the expensive III-V semiconductor material which is used mainly as filler to align the active circuits to the SI waveguide. The number of circuits/chips in the wafer is significantly lower than that of regular MMIC wafer since the MMIC circuit area is usually much smaller than the WG area. Second, the device yield is directly linked to the product of the yields of each wafer layer, therefore is affected greatly when the MMIC yield is low. If a functional low noise amplifier (LNA) is matched with a non-functional diode, the entire circuit is non-functional. This problem compounds at higher frequencies where yielding circuits becomes more difficult and testing becomes more costly. Third, in a WLP approach, when the device requires multiple technologies integrated together, the number of wafer layers increases and the device yield decreases quickly with the increasing layer and integration complexity.
Additional hurdles for WLP at sub-millimeter wavelengths are found in the semiconductor manufacturing steps required to achieve the required device density. A common technique used in fabricating semiconductor devices involves the use of photoresists (PR), for example, during the processing and/or patterning of semiconductor wafers. The wafers may be formed from materials such as silicon, III-V compounds, II-VI compounds, or others known to those skilled in the art. Wet photoresists (wet PR) are commonly used, but are often inadequate for patterning wafers that include deep trenches. For example, as trench depth increases to approximately 30-50 μm, the wet PR becomes more difficult to process with acceptable results.
Various issues may be present when using wet PR processes. In one example, sidewalls and/or the bottom of the trench are difficult to be fully covered by the wet PR, leaving them exposed to the subsequent process. Some techniques may help to alleviate these issues if the trench depth is close to, or less than approximately 30-80 μm. Examples of these techniques include multiple wet PR coatings and/or multiple wet PR exposures and development. However even with these techniques, it may be difficult to define accurate patterns and small features.
Another issue with wet PR is a non-uniform PR thickness. Wet PR is typically applied with a spin-coating process, but this commonly results in non-uniform thickness, especially near the edges of a trench. The wet PR thickness may be too thin in some areas while being too thick in other areas with an irregular or unpredictable relationship between coverage location and PR thickness. Where the PR is too thin, the patterns lose their accuracy easily after long or multiple exposures. Where the PR is too thick, it is difficult to fully develop the PR.
Finally, extremely thick wet PR may “pile-up” in some areas, especially when multiple wet PR coatings are used. This pile-up makes it very difficult to develop the wet PR even with multiple PR exposures and development. The pile-up also makes it difficult to define accurate and small features.